Classroom acoustics design for speakers comfort and speech intelligibility: a European perspective

Aalborg Universitet Classroom acoustics design for speakers comfort and speech intelligibility Garcia, David Pelegrin; Rasmussen, Birgit; Brunskog, Jonas Published in: Proceedings of Forum Acusticum 24 Publication date: 24 Document Version Publisher's PDF, also known as Version of record Link to publication from Aalborg University Citation for published version (APA): Garcia, D. P., Rasmussen, B., & Brunskog, J. (24). Classroom acoustics design for speakers comfort and speech intelligibility: a European perspective. In Proceedings of Forum Acusticum 24. European Acoustics Association - EAA. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.? You may not further distribute the material or use it for any profit-making activity or commercial gain? You may freely distribute the URL identifying the publication in the public portal? Take down policy If you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from vbn.aau.dk on: January 4, 26

Classroom acoustics design for speakers comfort and speech intelligibility: a European perspective David Pelegrin Garcia Acoustics and Thermal Physics, Katholieke Universiteit Leuven, Heverlee, Belgium. Birgit Rasmussen Danish Building Research Institute, Aalborg University Copenhagen, Denmark. Jonas Brunskog Acoustic Technology, Technical University of Denmark, Lyngby, Denmark Summary Current European regulatory requirements or guidelines for reverberation time in classrooms have the goal of enhancing speech intelligibility for students and reducing noise levels in classrooms. At the same time, school teachers suffer frequently from voice problems due to high vocal load experienced at work. With the aim of improving teachers working conditions, this paper proposes adjustments to current regulatory requirements on classroom acoustics in Europe from novel insights on classroom acoustics design that meet simultaneously criteria of vocal comfort for teachers and speech intelligibility for students. Two room acoustic parameters are shown relevant for a speaker: the voice support, linked to vocal effort, and the decay time derived from an oral-binaural impulse response, linked to vocal comfort. Theoretical prediction models for room-averaged values of these parameters are combined with a model of speech intelligibility based on the useful-to-detrimental ratio and empirical models of signal-to-noise ratio in classrooms in order to derive classroom acoustic guidelines, taking into account physical volume restrictions linked to the number of students present in a classroom. The recommended values of reverberation time in fully occupied classrooms for flexible teaching methods are between 5 s and s (between and.7 s in an unoccupied but furnished condition) for classrooms with less than 4 students and volumes below 2 m 3. When designing larger classrooms, a dedicated acoustic study taking into account considerations about geometry, material and speaker/audience placements can help to provide good speech intelligibility across the audience, while increasing the voice support and reducing the vocal effort. PACS no. 43.55.Fw, 43.55.Hy. Introduction The acoustic design of classrooms, both in terms of noise control and room acoustics, is relevant because it affects the quality of oral communication between teachers and students, which is still the most common way of teaching and learning. Students need a physical environment which preserves speech intelligibility, makes the oral message easy to listen and engages them into the process of learning. At the same time, teachers speech needs to be understood; they have to speak comfortably and without straining their voice. Traditionally, classroom acoustic guidelines and regulatory requirements have been designed based on (c) European Acoustics Association speech intelligibility considerations. The present paper is based on a recent publication [] and it summarises the results of past years research on teachers vocal comfort and effort in order to derive guidelines that meet simultaneously criteria of vocal comfort for teachers and speech intelligibility for students. This novel approach may be useful to update existing regulatory requirements on classroom acoustics. 2. Current European guidelines and regulatory requirements Classroom acoustics is a concern of increasing awareness, as demonstrates the increasing number of countries in Europe which have included classroom acoustic regulatory requirements in their building codes during the last 5- years (e.g. Spain) or which have

made these requirements more strict (e.g. Denmark and Norway). A summary of the requirements regarding reverberation time in ordinary classrooms for general primary and secondary education is shown in Table I. Some of the countries (Denmark, Norway, UK, Spain) establish maximum values for the reverberation time whereas others (Germany, France) establish target ranges as a function of the volume. In particular, Germany establishes a relationship between the volume and the target reverberation time T soll (soll is the German word for target) and defines a range of accepted values around that target reverberation time for different frequency bands. This is a trend that other European countries follow (e.g. Austria [2], Switzerland [3], Belgium [4]). The Norwegian Building Code expresses the acoustic requirements of classrooms as class C of Classification Scheme NS875:22 [8]. A Classification Scheme sets acoustic criteria to classify classrooms (and other spaces) in different levels of acoustical quality, called classes. In NS875:22, class A has the highest acoustic quality and class D has the lowest (Classification Schemes in other countries might have different criteria and denomination for the classes). In NS875:22, reverberation time in classrooms should be not higher than s to achieve class A or B,.5 s for class C or s for class D. Sweden and Iceland also have Classification Schemes. The trend in Classification Schemes is that classrooms are rated better the shorter the reverberation time. A more thorough presentation of classroom acoustic requirements and recommendations in European countries is found in []. Rasmussen et al. [] present a similar study focused on Nordic countries. 3. Room acoustic parameters for a speaker Room acoustics parameters that aim at characterising the propagation of sound between the mouth and the ears of a speaker are derived from an oral-binaural room impulse response (OBRIR), i.e. an impulse response measured at a microphone located at the end of the ear canal of a dummy head when a loudspeaker inside its mouth acts as the source. The derivations that follow assume the use of a head and torso simulator (HATS) from Brüel & Kjær type 428. Two parameters are found relevant: the voice support ST V and the decay time DT 4,ME. 3.. Voice support The voice support ST V is a measure of the degree of amplification of a room to the voice of a speaker at his own ears [2]. More specifically, it is defined as the difference between the reflected sound level (L R ) and the airborne direct sound level (L D ) of the voice of a speaker, as found in an OBRIR: ST V = L R L D (db). () The voice support is related to the vocal effort experienced by a speaker in different rooms [3 8], and is negatively correlated to the radiated voice power level L W. An empirical model that represents the average variations in voice power level L W of a speaker, with respect to the average voice power level used in freefield (L W, ), as a function of ST V [3, 4] when low background noise levels are present is L W = L W L W, = { 3.78 ST V (db), 4.5dB ST V 6.5dB.5 35 log ( ST V / + ) (db), ST V 4.5dB (2) A prediction model for ST V [2], averaged across positions in a room, is presented in Fig., considering a flat T 6 across frequency and a room of proportions 2.8:.6:. ST V decreases almost linearly with the logarithm of V (except for the largest volumes at low reverberation times) and increases with T 6. The axis on the right edge shows L W according to Eq. (2). 3.2. Decay time DT 4,ME The decay time DT 4,ME [8] (in which the subindex ME stands for Mouth-to-Ears) is defined as the time it would take for the backwards integrated energy curve of an OBRIR to decay 6 db after the arrival of the direct sound, calculated from the initial decay of 4 db (value of the subindex in DT 4,ME ) and assuming a linear decay. Differently from traditional impulse responses in which the receiver is far away from the source, in an OBRIR source and receiver are located very close to each other, so the direct sound has much more energy than the reflected sound. Therefore, the DT 4,ME is very sensitive to the direct-to-reflected sound level difference. In a previous study [8], it was found that the general sensation of vocal comfort Ĉ, a score given by talkers on questionnaires, is non-linearly related to DT 4,ME for talkers with healthy voices. A fitted quadratic regression model (R 2 = 5; p <.) is Ĉ = 6.4 DT 2 4,ME + 5.8 DT 4,ME., (3) The optimum vocal comfort Ĉ opt is located at DT 4,ME,opt 5 s. Three categories of vocal comfort are defined: recommended, acceptable, and nonacceptable. The recommended vocal comfort range is where the vocal comfort Ĉ is more than 7% of the optimum value, thus for DT 4,ME between.35 and.55 s. In the acceptable range, the vocal comfort is less than 7% of the optimum value but it is higher than the average ( Ĉ Ĉopt/ 2), which occurs at two different intervals: DT 4,ME from.25 to.35 s and from.55 s to 5 s. The non-acceptable range is given by a less than average vocal comfort (Ĉ ), which occurs for DT 4,ME <.25 s and for DT 4,ME > 5 s.

Table I. Regulatory requirements for classroom reverberation time in different European countries, in addition to details about the conditions of the criteria. Country Standard/ Guideline Year Required T [s] Occupancy Details of T Denmark BR 2 [5] 2 Furnished unoccupied France Arrêté du 25 avril 23 [6] Germany DIN 84:24 [7] Norway NS875:22 [8] Spain CTE DB-HR [9] 23 V < 25 m 3 : T V > 25 m 3 : T.2 24 T soll =.32 log V.7 m 3 (V = m 3 T soll = 7s V = 25m 3 T soll = s) Furnished unoccupied Fully occupied 22.5 (Class C in [8]) Furnished unoccupied 29 V < 35 m 3 :.5 Fully occupied UK BB93 [] 23 Nursery & primary: Secondary: Unfurnished unoccupied 25 Hz to 4 khz. Limit for each / octave band. 25 Hz: +2% accepted. Average 5 2 Hz Requirement for Hz to 5 khz. Frequency band values with ±2% tolerance in 25 Hz 2 khz, extending progressively to +2%/ 4% toward Hz and 5 khz. Unoccupied values +.2 s. 25 Hz to 4 khz. Limit for each / octave band. 25 Hz: +3% accepted. Average 5 2 Hz. Limit for empty unfurnished classroom: T <.7s Average 5 2 Hz. T may gradually increase at low frequencies (25 Hz and 25 Hz) but no more than 3% over limits Voice Support [db] 6 8 4 6 8..2.6 2..5.3 T 6 =.2 s 2 2 5 2 4 8 6 Figure. Average voice support versus room volume for different values of T 6, considering a flat T 6 across frequency. The axis on the right edge shows average voice power level variations according to Eq. (2). 8 7 6 5 4 3 2.5 Relative voice power level [db] classroom acoustic design and are shown in Fig. 2. The same figure shows also the target reverberation time which optimises the vocal comfort. A suitable empirical fit for the target reverberation time is T target,vc =.32 3 V +.38 (s), (4) In principle, the reverberation time in classrooms for optimum vocal comfort should be within the recommended range, i.e. for T target,vc T.2T target,vc,.35 s DT 4,ME.55 s. Moreover, Fig. 2 shows different lines for L W (at -, -3 and -5 db), obtained through the prediction model for ST V and Eq. (2). These lines are correlated with the vocal effort in silent conditions. The more negative L W, the better for keeping a low vocal effort. The line L W = 3 db should be considered as a reference for a classroom that does not put high demands on teachers voices. Classroom acoustic designs at the left of this line in Fig. 2 should be sought. A prediction model for the room-averaged DT 4,ME [8] as a function of the reverberation time and the volume in the room, as an average in the octave bands of 2 khz and 4 khz, considering a flat T 6 across frequency and a room of proportions 2.8:.6:, is shown in Fig. 2. In typical rooms, DT 4,ME is shorter than T 6 and decreases with V. 3.3. Optimisation of speakers parameters The recommended and acceptable categories of vocal comfort, defined by the ranges of DT 4,ME obtained in the previous section, provide a first guideline for 4. A prediction model for speech intelligibility The useful-to-detrimental ratio U 5 is a suitable measure of speech intelligibility [9]. In this case, the useful part L SL,early is the sound signal arriving in the first 5 ms after the arrival of the direct sound (including it) and the detrimental part L SL,late+noise contains the late reflections (arriving later than 5 ms after the direct sound) and the noise. Thus, U 5 = L SL,early L SL,late+noise (db). (5)

.6.4.2.2 Acceptable comfort Recommended comfort T target,vc optimum comfort Equal L W.9 s.7 s 5 db 3 db. s L W = db.5 s.3 s DT 4,ME =. s 5 2 4 8 6 Figure 2. Predicted DT 4,ME and L W (derived from ST V ), as a function of reverberation time and volume. The areas of DT 4,ME providing recommended and acceptable voice comfort levels are indicated in darker and lighter gray color. Bold black line: target reverberation time T target,vc which maximises vocal comfort. Bold gray lines: Equal L W curves derived from ST V model According to Nijs and Rychtarikova [9], L SL,early is given by L SL,early = L W + log ( Q 4( ᾱ)βfb r/lmfp + 4πr2 ᾱs ( ) ) e 9/T 6 (db), (6) where L W is the radiated sound power level, Q is the directivity of the speaker in the direction aiming at the listener, r is the distance between speaker and listener, ᾱ is the mean absorption coefficient of the room, β fb is an experimental parameter related to the spatial decay rate of the reverberant field and l mfp is the mean free path of the room (l mfp = 4V/S). It is assumed that the reflected sound follows an exponentially decaying random process with an amplitude that decays with longer distances to the source, as suggested by Barron in the context of acoustics for performance spaces (without any parameter β fb, i.e. β fb = ) [?] and experimentally refined by Sato and Bradley [2] in the context of classroom acoustics by introducing the parameter β fb, usually >. The late arriving speech level L SL,late, on the other hand, contains the effect of sound reflections arriving later than 5 ms after the arrival of the direct sound and the signal-to-noise ratio (SNR): L SL,late+noise = L W + ( 4( ᾱ) β fb r/l mfp log ᾱs e 9/T6 + 4 ) SNR/ ᾱs (db), (7) A formal definition of SNR is the difference between the sound power levels of speech L W,speech and student-activity noise L W,SA, SNR = L W,speech L W,SA (db). (8) Empirical models that link A-weighted studentactivity noise level L SA, instructor voice power levels L W,speech, volume V, acoustic absorption of the room A (A = Sᾱ)and the number of students N in university classrooms are given by Hodgson et al [2]. The inputs to the models were measurements during 8 lectures in classrooms with V between and 957 m 3, between 6 and 254 students present in the classroom, and total occupied absorption areas between 3 and 35 m 2. The A-weighted student-activity noise level L SA is described as: L SA = 83 + log N 34.4 log A +.8A (db re 2µPa). (9) The A-weighted student-activity noise power level L W,SA is ( ) 4( ᾱ) L W,SA = L SA log Sᾱ (db re pw), () where it is assumed that L SA is homogeneous throughout the classroom. The A-weighted sound power levels of speech L W,speech, averaged for male and female teachers, is [2]: L W,speech = 53.5 +.5 L SA +.6 V 9.6 log A (db re pw). () Using Eqs. (8) () for SNR, the resulting U 5 depends on the number of students N. Figure 3 shows the U 5 values as a function of the volume, the reverberation time and the number of students (N =, 2, 4 and 8). The horizontal axis has been adjusted to display volumes of at least 4 m 3 per student. For simplicity, it is assumed that the speaker can be oriented toward any direction (Q = ), that the speakerto-listener distance is the geometrical mean of the length and the width of the room, and that β fb =. In the figure, the equal U 5 contours are shown in steps of db. In the same figure, the upper and lower limits of reverberation time that result in recommended vocal comfort levels are shown with bold dashed lines. The hashed areas in Fig. 3 indicate areas for which the prediction of SNR is not valid due to unavailable data for the model. The reverberation time curve that defines maximum U 5 as a function of the volume is close to the lower limit of recommended vocal comfort zone (T target,vc ) for or 2 students. This curve increases with the number of students and falls

.6.6 N = N = 2-3 db.4.4.2.2 2 db Maximum U 5 (V) -4 db U 5 = db db db db Limits recommended vocal comfort 2 db 5 2 3 4 6 8.2.2 db U 5 = db -4 db Maximum U 5 (V) db db 5 2 3 4 6 8.6 N = 4-3 db.6 N = 8-3 db.4.4.2 U 5 = db Maximum U 5 (V) db db.2 Maximum U 5 (V) U 5 = db -3 db -4 db.2-4 db.2 2 3 4 6 8 4 6 8 Figure 3. U 5 as a function of room volume and reverberation time, considering typical SNR measured in classrooms, for N = students (top left), N = 2 (top right), N = 4 (bottom left) and N = 8 (bottom right). The upper and lower limits of reverberation time to achieve recommended vocal comfort levels are indicated in bold dashed lines. Bold dotted lines indicate reverberation times that maximize U 5 for each volume. Hatched areas correspond to combinations of volume and reverberation time for which the SNR predictions are not valid. within the recommended range of reverberation time that optimises vocal comfort. None of the U 5 values in Fig. 3 exceeds 3 db (i.e. STI > 5), even with a low number of students. Nevertheless, the student-activity noise levels used for the SNR calculations are median levels of the periods when the teacher is not talking. It is likely that these levels are an overestimate of the actual noise levels present while the teacher is talking. 5. Guidelines for classroom acoustic design A classroom acoustic design that maximises simultaneously criteria of vocal comfort and speech intelligibility is found as the intersection of areas of recommended vocal comfort (T target,vc T.2T target,vc ) and the areas that provide speech intelligibility higher than a minimum value U 5 U 5,min. An additional requirement of L W < 3 db is set to limit vocal effort in silent conditions. If, for example, U 5,min is taken as.5 db (STI > ), the overlap between areas decreases with the number of students, and there is no overlap for 4 students. This overlap is summarised in Fig. 4(a) as target values of V and T in occupied classrooms as a function of N. The area of recommended design decreases with N. Beyond N = 2 students, speech intelligibility criterion becomes too restrictive. In this case, the best acoustic conditions for small groups of students (up to 2 pupils) are obtained for T values around 5 s (and volumes up to 2 m 3 ) in occupied classrooms. The choice of.5 db for U 5,min leads to small groups of students, which is advisable for groups with special needs (e.g. students with hearing impairment or non-native speakers). A more realistic choice of U 5,min = db (STI >.55) is used to derive Fig. 4(b), which shows the combinations of volume

.9 Occupied furnished classroom U 5 >.5 db.9 Occupied furnished classroom U 5 > db.7.5 2 5 N 5.7.5 25 3 N 4 35 35 3 (a) 5 2 25 3 35 4.9 Unoccupied furnished classroom U 5 >.5 db (b) 5 2 25 3 35 4.9 Unoccupied furnished classroom U 5 > db.7 5 5 N 2.5 (c) 5 2 25 3 35 4.7 35 N 4 3 25 2.5 5 5 (d) 5 2 25 3 35 4 Figure 4. Areas of recommended volume and reverberation time in occupied (a,b) and unoccupied (c,d) classrooms, for a given number of students N, which satisfy simultaneously criteria of vocal comfort and speech intelligibility for U 5,min =.5 db (a,c) and U 5,min = db (b,d). For each N, the design area should be considered as the interior of the boundary line. and reverberation time that provide simultaneously good vocal comfort and satisfactory speech intelligibility (U 5 > db) for any number of students up to 4. For a classroom for flexible teaching with 4 students, design possibilities become restricted to reverberation times between 5 and s and volumes between 6 and 2 m 3. For less students, volumes are restricted and depend on T. For T = 5s, the maximum volume is 2 m 3, and for T =.7s, it is approximately 35 m 3. Whereas the derivation of guidelines has been suggested for fully occupied conditions, regulations in classroom acoustics are most often referred to unoccupied but furnished conditions. In an occupied classroom with volume V, total surface area S and reverberation time T occ, the mean absorption coefficient ᾱ occ according to Eyring s formula is ᾱ occ = exp [.6V/(T occ S)]. (2) In order to obtain an estimate of the acoustic conditions in unoccupied furnished classrooms, the absorption corresponding to the students and the teacher, considering that each person in the classroom absorbs A s =.28 m 2 (according to Sato and Bradley [2]), is subtracted from the total absorption area in the occupied classroom A occ = Sᾱ occ. Thus, the mean unoccupied absorption coefficient ᾱ unocc results in ᾱ unocc = A occ (N + )A s, (3) S which leads to a reverberation time in unoccupied but furnished conditions T unocc of T unocc =.6V S ln( ᾱ unocc ). (4) Each of the curves limiting an area of recommended design in Figs. 4(a-b) is transformed into a curve which expresses the recommended design areas of volume and reverberation time in unoccupied furnished classrooms for a number of students by applying Eqs. (2) (4). Figures 4 (c-d) show the areas of recommended design in unoccupied but furnished classroom for a number of students between a minimum of N min = 5 and a maximum N.

According to Fig. 4(c), reverberation times in unoccupied furnished classrooms designed for high speech intelligibility should not exceed.7 s in any case, whereas the minimum recommended reverberation time depends on the number of students. Such a classroom with up to 2 students should have a T unocc of around.55 s. In classrooms with less strict speech intelligibility requirements, as shown in Fig. 4(d), the minimum T unocc for classrooms with N 2 students is slightly higher than.5 s, whereas the minimum T unocc for classrooms with N 4 students is s. 6. Concluding remarks In classrooms for flexible teaching methods with no more than 4 students, reverberation times above s (in conditions of full occupancy) or.7 s (in unoccupied furnished classrooms) should be avoided. Volumes should be kept no larger than 2 m 3 for the highest occupancy (paradoxically, volumes can be larger for less students). Higher reverberation times would decrease speech intelligibility, which at the same time would disengage students from learning. Whereas some guidelines, like the one proposed by the BATOD [22], stand for reverberation times shorter than s based on speech intelligibility considerations, the present study suggests that reverberation times slightly higher (between 5 s and.5 s in occupied classrooms) might be optimal, and that lower reverberation times are not necessarily better, based on vocal comfort considerations. Furthermore, reverberation times lower than.3 s might generate overdamping, i.e. an excessive attenuation of speech levels at the last rows [9]. Therefore, guidelines should include target reverberation times (or upper and lower limits) rather than maximum allowed reverberation times, and should include limitations in volume and number of students at least regarding the necessity of having a dedicated study with geometrical considerations and distribution of absorbing material on the surfaces of the classroom, if designing larger classrooms for more students. In some European countries (see Table I), like France or UK (for secondary schools), reverberation times up to s in unoccupied spaces are allowed, which may be detrimental for speech intelligibility if, with students present, the reverberation time does not descend below s. On the other hand, in Norway, reverberation times in empty furnished classrooms are limited to.5 s ( s for the highest quality class in the classification scheme), which might be too restrictive and lead to low vocal comfort levels. From the studied countries, only Germany and France set maximum and minimum limits to reverberation times. Regarding volume limitations, France includes the prescription of a dedicated study for classrooms larger than 25 m 3, Spain for classrooms larger than 35 m 3 and Germany gives special design guidelines for classrooms larger than 25 m 3. According to the present findings, it is apparent that some guidelines should be adjusted: Denmark, Norway, Spain and UK should add minimum limits for their required reverberation times. France and UK (for secondary schools) should lower the maximum allowed reverberation time. Spain should lower the volume of 35 m 3 for the need of doing a dedicated study. Moreover, Denmark, Norway and UK should take the volume of the classroom into consideration. Classification standards, such as the Norwegian NS875:22 should also be revised to consider what are the best quality standards and include target reverberation times instead of upper limits. According to the present findings, a limitation of the maximum reverberation time to s in furnished but unoccupied classrooms for the highest quality classes is detrimental for vocal comfort. Current European regulations differ in the methods they use to express the requirements in terms of reverberation time, volume, occupancy conditions and additional demands on the frequency dependency and validation of the design. With the existing knowledge in classroom acoustics, there is an excellent opportunity to work towards the harmonisation of regulations among European countries. References [] D. Pelegrin-Garcia, J. Brunskog, and B. Rasmussen, Speaker-oriented classroom acoustics design guidelines in the context of current regulations in european countries, Acta Acust. united Ac., 24. Accepted for publication. 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